The invention relates to a photopolymer formulation comprising a polyol component, a polyisocyanate component, a writing monomer and a photoinitiator containing a coinitiator and a dye of formula F+An−, where F+ represents a cationic dye and An− represents an anion. The invention further relates to a holographic medium, particularly in the form of a film, containing a photopolymer formulation according to the invention, to the use of such a medium for recording of holograms, and also to a specific dye usable in the photopolymer formulations according to the invention.
Photopolymer formulations of the type mentioned at the beginning are known in the prior art. WO 2008/125229 A1, for instance, describes a photopolymer formulation comprising a polyol component, a polyisocyanate component, a writing monomer based on acrylate and also photoinitiators containing a coinitiator and a dye. In the cured state, the writing monomer and the photoinitiators form a spatially isotropic distribution embedded in the polyurethane matrix formed from polyol and polyisocyanate components.
The uses of photopolymer formulations are decisively determined by the refractive index modulation Δn produced in the photopolymer by holographic exposure. In holographic exposure, the interference field of signal light beam and reference light beam (in the simplest case, that of two plane waves) is mapped into a refractive index grating by the local photopolymerization of, for example, high refractive index acrylates at loci of high intensity in the interference field. The refractive index grating in the photopolymer (the hologram) contains all the information of the signal light beam. Illuminating the hologram with only the reference light beam will then reconstruct the signal. The strength of the signal thus reconstructed relative to the strength of the incident reference light is diffraction efficiency, DE in what follows.
In the simplest case of a hologram resulting from the superposition of two plane waves, the DE is the ratio of the intensity of the light diffracted on reconstruction to the sum total of the intensities of the incident reference light and the diffracted light. The higher the DE, the greater the efficiency of a hologram with regard to the amount of reference light needed to visualize the signal with a fixed brightness.
When the hologram is illuminated with white light, for example, the width of the spectral range which can contribute to reconstructing the hologram is likewise only dependent on the layer thickness d. The relationship which holds is that the smaller the d, the greater the particular acceptance widths. Therefore, to produce bright and easily visible holograms, it is generally desirable to seek a high Δn and a low thickness d while maximizing DE. That is, increasing Δn increases the latitude to engineer the layer thickness d without loss of DE for bright holograms. Therefore, the optimization of Δn is of outstanding importance in the optimization of photopolymer formulations (P. Hariharan, Optical Holography, 2nd Edition, Cambridge University Press, 1996).
In order that a very high Δn and DE may be realized for holograms, the matrix polymers and writing monomers of a photopolymer formulation should in principle be chosen such that there is a very large difference in their refractive indices. One possible method of realization is to use matrix polymers having a very low refractive index and writing monomers having a very high refractive index. Suitable matrix polymers of low refractive index are for example polyurethanes obtainable by reaction of a polyol component with a polyisocyanate component.
In addition to high DE and Δn values, however, another important requirement for holographic media from photopolymer formulations is that the matrix polymers be highly crosslinked in the final medium. When the degree of crosslinking is too low, the medium will lack adequate stability. One consequence of this is to appreciably reduce the quality of holograms inscribed in the media. In the worst case, the holograms may subsequently even be destroyed.
It is further very important for the large scale industrial production of holographic media from photopolymer formulations in particular that the matrix polymers crosslink quickly. Short cure times to blocking resistance are very important here, since this parameter determines the processing speed and/or the length of any curing sector needed.
However, it has been determined that media obtained from the known photopolymer formulations are frequently devoid of adequate crosslinking. Moreover, reaching an even just adequate crosslink density requires long cure times in many cases. This means that media obtained from the known photopolymer formulations may have quality issues and the prolonged cure time is associated with appreciable cost and inconvenience in large scale industrial manufacture.
The problem addressed by the present invention was therefore that of providing a photopolymer formulation of the type mentioned at the beginning, from which stable holographic media for bright holograms are obtainable quickly and at low cost and inconvenience.